Thermosensitive organic material



Aug. 5, 1958 J. ROSENBERG 'I'HERMOSENSITIVE ORGANIC MATERIAL 3 Sheets-Sheet 1 Filed llay 31, 1957 IMPEDA E TEM PER URE WIRE INSULATION NYLON WI RE f/rre/rfar Jase i fasefrbefg f flaw-14rd /7/3 4292/0194 EmETxzo o; 3232! Aug. 5, 1958 J. ROSENBERG THERMOSENSITIVE ORGANIC MATERIAL Filed llay 31, 1957 '1. tzansxz: 1111112.! ZIIIIIIIIIIIISEIE m 3 Sheets-Sheet 3 firm/0r 1 M05406 Fwmbe g mu 4. inward.

fiZs 4/70/7750 THERMOSENSITIVE ORGANIC ATERIAL Joseph Rosenberg, Erie, Pa., assignor to General Electric Company, a' corporation of New orlr.

Application May 31, 1957, Serial No. 662,792

6 Claims. (Cl. 219-46) This invention relates to a composition of matter useful as electrical insulation which becomes increasingly conductive as the temperature thereof is increased. More particularly, the invention relates to a heater wire and automatic blanket incorporating the composition of matter therein.

A number of normally insulating organic composi- Patented Aug. 5, 1958 Fig. 5 is a graph on semi-log scale showing impedance versus temperature of one of the materials (55% by weight of Z-ethylhexyl acrylate and 45% by weight'of acrylonitrile to which has been added 1% by weight of 5 a cationic surface active agent) of the present invention as compared with nylon; Fig. 6 is a graph on semi-log scale showing the specific impedance -versus temperature of a composition (60% by weight of butyl acrylate and 40% by weight of acrylonitrile) of the present invention with several different types of surface active additives;

Fig. 7 is a graph on semi-log scale similar to Fig. 6 except that the composition comprises approximately 45 acrylonitrile and 55% 2-ethylhexyl acrylate; Fig. 8 is a graph on semi-log scale comparing the impedance change 16 with temperature change for a number of different compositions of the present invention with respect to nylon; Fig. 9 is a graph illustrating the effect of moisture on the compositions of this invention as compared with the effect moisture has on nylon; Fig. 10 is a representation of a typical electric blanket shown folded; and'Fig. 11

tions are known which have a negative coeliicient of thermal impedance or resistance whereby the electrical conductivity of the composition increases by several orders of magnitude as the temperature increases within a relatively narrow range, i. e., 50 C. This property has been utilized to actuate a heat control circuit in an automatic electric blanket as disclosed in Spooner and Greenhalgh Patent No. 2,581,212, which is assigned to the same assignee as the present application. In Patent No. 2,581,212, this control function was exercised by the use of nylon as an insulating material, although other organic materials, namely, cellulose esters and vinyl halide resins, were set forth as satisfactory but less desirable substitutes for nylon. Reference is also made to Jacoby, Becker, and Rosenberg application Serial No. 662,793, filed of even date herewith and assigned to the same assignee as the present application, which is directed broadly to additives which affect the coeflicient of thermal impedance.

While the electrical characteristics of nylon have proved suitable for effecting overheat protection in automatic electric blankets, thematerial herein disclosed has proven to have superior heat aging characteristics and to be less sensitive electrically to moisture.

It is one object of this invention to provide a composition of matter having a high negative coeliicient of thermal impedance.

It is another object of this invention to provide a heater wire insulated with a composition of matter allowing satisfactory temperature control functions while maintaining resistance to heat aging and moisture sensitivity.

It is a further object of this invention to provide an automatic blanket, or the like, in which the temperature control function is exercised through a composition of matter having a high negative coefiicient of thermal impedance which blanket can be used over a long period of time without having the composition of matter degrade substantially.

Other objects of the invention will be apparent from the following specification considered in conjunction with the annexed drawing wherein Fig. 1 is an enlarged elevntion, partly in section, showing one form of temperature-sensitive element embodying this invention; Fig. 2 is similar to Fig. 1, showing a second structural form thereof; Fig.3 is a control circuit diagram associated with the construction of Fig'. 1, said circuit utilizing the inner conductor of the thermosensitive element as the 1oadfor example, the heater conductor of an electric blanket; Fig. 4 is another control circuit, in which the load is independent of the temperature-sensitive element;

is a schematic view showing the distribution of heater wires in an electric blanket.

Briefly stated, in accordance with one of its aspects,

this invention is directed toward a composition of mattercomprising, a copolymer of acrylonitrile and an acrylic erties over a long period of time andin the presence of moisture, enables them to be used as insulation and for temperature control purposes in automatic electric blankets and other types of electric resistance heaters. Overheat protection for electric motors, cable, and other electrical apparatus may also be obtained by utilizing this composition of matter.

While the present invention is directed broadly to copolymers of acrylonitrile and acrylic acid esters in which there is incorporated up to 5% of a surface active agent, it is preferred that the copolymer contain between 30% and 85% acrylonitrile and optimum results are achieved within the range of 35% to acrylonitrile. While acrylic acid esters are generally satisfactory, best results have been obtained with alkyl esters with a chain length of between 4 and 12 carbon atoms. Preferred esters within this range are Z-ethylhexyl acrylate and n-decyl acrylate. However, it is to be emphasized that satisfactory results have been obtained with other acrylates such as ethyl acrylate, l'auryl acrylate, and cetyl acrylate.

The surface active agents which may be used to enhance the change in impedance with temperature can be cationic, nonionic, or anionic. A preferred cationic material is stearyldimethylbenzyl ammonium chloride, commonly sold under the trade' name Triton K 60, but other surface active materials which are satisfactory are diisobutylphenoxyethoxyethyldimethylbenzyl ammonium chloride, commonly sold as Hyamine 1622, and methyldodecylbenzyltrimethyl ammonium chloride, commonly anionic surface active agent is the sodium salt of alkylarylpolyether sulfate, commonly sold as Triton 770. Typical nonionic materials are the alkylarylpolyether alcohol, commonly sold as Triton' X100, and the sodium salt of alkylarylpolyether sulfonate, commonly sold as Triton X-200. While the above are typical examples of surface active agents, it is to be emphasized that they are representative rather than limiting. Such surface active agents have been known for many years as emulsifiers, but their use as additives to effect an enhanced change in impedance with temperature change has not been previously reported. Obviously, the proportion of surface sold as Myamine 2389. An example of a satisfactory An example of emulsion polymerization (in a sealed vessel) The following charge is placed in a 1,000 mi. bottle:

2-ethylhcxyl acrylate (inhibitor free) "grams" 35.0 Acrylonitrile (inhibitor free) do 60.0 Stcaryldimethylbenzyl ammonium chloride (Triton K. 60) grams 5.0 Potassium pcrsulfate do 0.1 Lauryl mercaptan do 0.1 Distilled water ml 400.0

The potassium persulfate catalyst is added last. The

bottle is shaken briefly and then placed on the rotating rack in a water bath at 80 C. for 15 hours. The reaction mixture is then steam distilled and precipitated in 2 liters of 5% sodium chloride solution. The precipitate is washed free of chloride ion and then dried.

EXAMPLE 2 An example of emulsion polymerization (in an open vessel) A 12 liter flask is set in a steam bath and the following charge added:

n-butyl acrylate (inhibitor free) grams 585 Acrylonitrilc (inhibitor free) do 900 Methyldedecylhenzyltrimethyl ammonium chloride (Hyamine 2389) "grams..- l5 Lauryl mercaptan do 1.5 Potassium persulfate do 1.5 Distilled water n1l 6000 The flask is equipped with a thermometer, stirrer, and four 400 ml. condensers (two in a joint). The contents are subjected to constant vigorous stirring. The mixture is heated with utmost caution until the first sign of a vigorous reaction is observed. The heat input is then decreased and the reaction allowed to go on under its own heat of reaction (heat of polymerization). After the initial reaction subsides, the heat input is once more cautiously raised and the reaction continued to completion. The mixture is then steam distilled and precipitated by pouring it into 20 liters of 5% sodium chloride solution. The precipitate is then washed free of chloride ion and dried to constant weight. The normal yield is about 83% or higher.

EXAMPLE 3 An example of suspension polymerization Forty grams of n-decyl acrylate, 60 grams of acrylonitrile, 6 drops of a solution consisting of the sodium salt of an alkylarylpolyether sulfate (Triton 770), and.

will be completed in the first half-hour after initiation of polymerization, it is desirable to maintain the temperature of the reaction mixture for an additional two to three hours in order to insure a complete reaction.

. '4' After this period, the mixture is steam distilled, filtered, washed, and dried in a vacuum oven to constant weight. The normal yield is about 88%.

While the above examples are directed specifically to Z-cthylhexyl acrylate, n-butyl 'acrylate, and n-dccyl acrylate, it should be understood that they are equally applicable to other acrylates such as ethyl acrylate, nhexyl acrylate, cyclohexyl acrylate, Z-ethylbutyl acrylate, n-octyl acrylate, lauryl acrylate, and cetyl acrylate.

In the polymerization processes set forth in Examples l-3, the surface active agent served primarily as an emulsifier which was eliminated during the processing. Accordingly, during the process of mixing and milling the copolymer prior to extrusion, surface active agent may be added when it is desirable that it bepresent in the final product. The steps of mixing, milling, and extruding are conventional. In general, it has been found that the best milling temperature ranges from 110 C..to 170 C., and the best extrusion temperature is about -40 degrees above the mill temperature. In the extruder, a difference of 20 to 30 C. between the higher head temperature and the lower barrel temperature helps to give good flow.

The physical appearance and the color of extruded wire are good indicators of whether the material has been extruded properly. When the temperature is extremely low, the material extruded is fibrous in character and may be pulled apart easily. A temperature too low but slightly nearer the proper extrusion temperature produces a striated effect on the surface of the resin. A more obvious effect is the appearance of lumps when the temperature is insufficient to soften the material. A high temperature which is fairly close to the proper extrusion temperature gives a high gloss to the polymer. At too high a temperature, irregular flow lines appear.

The graph of Fig. 5 compares the impedance-temperature characteristics of wire insulated with nylon to the same wire insulated with a copolymer comprising 45.4 parts by weight acrylonitrile and 54.6 parts by weight Z-ethylhexyl acrylate to which has been added 1% of the cationic surface active agent previously referred to as Triton K-60. From this graph, it may be seen that in the temperature range of C. to 100 C. nylon declines in impedance from approximately 4.0Xl0 ohms to l.4 (l0 ohms per cm., or a ratio of approximately 3 to 1 whereas the copolymer declines from approximately 2.3 10 ohms per cm. to 0.38 ohm per cm., or a ratio of approximately 6 to 1.

The graph of Fig. 6 illustrates the etfeet of surface active agents on the change in specific impedance with temperature. In all cases, the copolymer was composed of 40 parts by weight of acrylonitrile, parts by weight of n-butyl acrylate, and I part by weight of surface active agent in the case of the three samples which contain such an additive. Within the temperature range of 50 C. to C., the polymer without surface active agent does not decline sufficiently in specific impedance to enable a satisfactory control function to be exercised. Within this same range, the polymer with anionic surface active agent added thereto declines from about 3.1)(10 ohms to 0.63Xl0 ohms for a ratio of about 5 to 1. This is sufficient for such polymer to be used in effecting temperature control. In the case of the polymer with a nonionic surface active agent additive, the specific impedance declines by a ratio of about 10 to l, and in the case of the polymer with a cationic surface active agent additive, the specific impedance declines by still a larger ratio.

The graph of Fig. 7 is somewhat similar to the graph of Fig. 6 except that the polymer was composed of approximately 45 parts by weight of acrylonitrile, 55 parts by weight of 2-ethylhexyl acrylate, and 1 part by weight of surface active agent in the case of the two samples which contain such an additive. In Figs. 6 and 7, the addition of a nonionic surface active agent produces a sharp decline in specific impedance with a rise in temperature, and in the case of the addition of a cationic surface active agent, the rate of decline shows a further marked increase.

The graph of Fig. 8 shows a comparison of impedance of acrylonitrile copolymers and nylon on wire. The range of impedance coefficients that is possible with the various copolymers and with various surface active agents is clearly illustrated.

It has heretofore been mentioned that one of the disadvantages of nylon is its change of characteristics in the presence of moisture. In the graph of Fig. 9, the comparison of the effect of humidity on the impedance of acrylonitrile-acrylate copolymers and nylon is illustrated. In all cases, the tests were run on wire insulated with the material and the curve for acrylonitrile-acrylate copolymers was the result of a compilation of data with respect to the copolymers of Figs. 6 and 7 as well as others. As may be seen, the curve for nylon is quite irregular while the curve for the copolymers 1S very nearly linear. It is obvious that constant impedance under changing humidity-constant temperature conditions is very important if a control function based upon change in impedance with temperature is to be exercised with a high degree of reliability.

Figs. 1 and 2 show typical constructions of combined flexible heater and thermosensitive elements embodying the present invention. In Fig. 1, the structure 10 includes preferably a ribbon-like bare conductor 11 wound upon a flexible strand 12 of fiber glass, stranded cellulose acetate, or other suitable flexible insulation. Over the conductor in intimate contact therewith, there is provided, as by extrusion, a layer or film 13 of copolymer of acrylonitrile and ester of acrylic acid of the type previously described herein. Wound tightly on the layer 13 is a bare conductor 14, also advantageously ribbonlike. One alternative, is to omit layer 13, but to wind conductor 14, previously coated with copolymer, directly on conductor 11. A water-inhibiting coating 15 of polyethylene may be applied over the conductor 14, although this is rendered unnecessary with the described copolymers, and over the layer 15 may be applied an outer insulating layer 15a which may be composed of polyvinyl chloride or similar material having good qualities of insulation, abrasion resistance, and the like. When the wire is to be used where there is no repeated flexing, a conventional solid or stranded conductor may be used and the core 12 eliminated. Other wire designs, making effective use of the copolymer systems described herein, may also be used.

In Fig. 2, the combined heater and thermosensitive element 10a has bare wires 16, 17, corresponding in function to the ribbon-like conductors 11 and 14 of Fig. 1, and tightly wound in parallel spaced relationship on a flexible insulating strand 18. Over this, there is extruded a layer of acrylonitrile-acrylate copolymer insulation 19 to substantially envelop the wires. The mass 19 serves to secure the wires 16 and 17 in fixed spaced relationship. In the embodiment of Fig. 1, the active thermosensitive material of the layer 13 is represented by its radial thickness, whereas in Fig. 2 the effective resistance material comprises an inner layer extending between the adjacent turns of wire 16, 17. It is preferred to cover the paired wire 16, 17 with the common layer of thermosensitive material, rather than to have one wire coated therewith and an adjacent bare wire laid tightly thereagainst, because of the surer physical contact of each wire with the thermosensitive layer.

In Fig. 2, the wall thickness of layer 19 is not critical except insofar as it interposes thermal insulation between the atmosphere and the inner thermosensitive layer between the spaced conductors, and it is entirely conceivable that the layer 19 may be suitably thick to act as a protective cover for the structure, in which event an additional insulation layer 20 may be dispensed with.

The active body of the thermosensitive material may conveniently be designated the control layer." As previously mentioned, the material of the control layer must exhibit a change in temperature, a substantial and predictable change in one or more electrical characteristics determining the electrical conduction of the layer and being capable of translation into a useful control effect. These electrical characteristics, for example, include significant change of direct or alternating current resistance, capacitive reactance, and impedance. One or more of such changes may be used in a control circuit to achieve the desired objective. The change should occur sharply within the temperature range for which the material is to function and be of a substantially greater order of magnitude than changes resulting from atmospheric conditions, surface cleanliness, and the like. The extent of change should provide a substantial distinction between the total value of the chosen electrical chaarcteristics of the whole element at its normal temperature and the total value of this characteristic when a relatively small link of the element is subjected to the control level of temperature. Further, the materials must be physically stable within the temperature range to be undamaged thereby and to resume their initial electrical characteristics upon return to normal temperature levels. It will be apparent that structural dimensions of the several component parts illustrated in Figs. 1 and 2 will be governed largely by usage factors incident to a particular end purpose. Used in electrically heated blankets, the core 12 may have a diameter of .020"; the inner conductor may be .008 in Width by .002" thickness, wound 35 turns per inch of core length; and the outer conductor may be .015 in width by .002" in thickness, wound 35 turns per inch of length.

In the embodiment of Fig. 2, the wires 16, 17 may be 36 AWG, and the core 18 .020" in diameter. In Fig. l, the control layer 13 may be as thin as consistent with its function as a tenacious, flexible, normally insulating film devoid of breaks or cavities. In the typical usage exemplified above, in which the line voltages will be of the order of volts A. C. and wherein the thermosensitive material hretofore described is used, a radial wall thickness of about .007" to .011" of the thermosensitive insulation provides adequate electrical and structural characteristics, and quick temperature response. The total diameter of the structures of Figs. 1 and 2 may be held in commercial production to below 0.1".

Figs. 3 and 4 exemplify applications typical of use of the invention in a safety or overheat cutout circuit for protection of electric blankets against scorching. The circuits shown therein are predicated upon the Crowley Patent No. 2,565,478, granted August 28, 1951. In Fig. 3, the inner conductor 11 of the thermosensitive element is used as the blanket heater, and the outer conductor 14 as the signal wire. The temperature of the blanket here shown is normally controlled in relation to room temperature by a cycling control device described and claimed in the United States patent to William K. Kcarsley, No. 2,195,857, April 2, 1940. It will be understood that in a blanket the element 10 preferably will be distributed in a series of convolutions over the blanket area to be heated, and may be run in passages provided for it as described in Patent No. 2,203,918, issued June ll, 1940, to I. O. Moberg. As shown in Fig. 3, the electrical system within the blanket 22 may be connected by the multiterminal plug and socket 23 to the control system, which is housed Within a control box 24 (see also Fig. l0). Plug P affords connection to the power source, for example the conventional 115 volt, 60 cycle, A. C. domestic circuit. In Fig. 3, the load circuit includes the conductor 11 of the element 10 arranged in two sections, as shown, to be connected in parallel to the power source; the parallel resistance of heater 11 will be of the order of 65 ohms. The outer conductor 14 is the overheat protective signal wire, and the control layer 13 may be a .007 inch film of one of the copolymer materials previously noted. Overheat protection is afforded by utilizing the change in impedance of the copolymer material as the electrical responsive characteristic at the control temperature level, and is provided through the medium of a work device such as a lockout relay and the cycling control, responsive to room temperature, of the aforementioned Kearsley patent is provided by the bimetallic switch 26 having a suitable external adjustment knob 27. The operating coil of the relay 25 is connected across an impedance bridge consisting of 0.1 mfd. capacitor 28 in a resonance circuit with a 75 henry choke 29, the other two legs of the bridge being the resistors 30, 31, of about 6800 ohms each. The coil impedance of relay 25 should be of the order of 90,000 ohms. The induced voltage of the resonant circuit, available at relay 25, is approximately 120 volts; the relay will pull in somewhat below that level, but will drop out at about 75 volts. Assuming that the cycling control 21 is calling for heat and its contacts are closed, power can be applied to the blanket heater wire 11 by momentarily closing the normally open switch 32 which will complete a circuit through power conductor 33, both branches of heater wire 11, conductor 34, 12,000-ohm resistor 35, switch 32, and power conductor 36. The signal wire 14, which in this instance preferably has a resistance of about 400 ohms, is in series with the center of the resonance circuit of choke 29 and capacitor 28, being connected thereto by the conductors 37, 38, which terminate suitably at plug 23. The relay 25 will pull in to bridge its contacts 39 and complete the operating circuit for the heater wire. The resistors 30, 31 provide a voltage divider circuit such that if a dead short occurred across the extremities of conductors 11, 14 while relay 25 was in, the voltage in the relay coil would drop to about one half line voltage, whereupon the relay would drop out and open the load circuit. In normal operation, both the resonant circuit and the power circuit are completed through conductor 40, relay contacts 39, and conductors 41, 36. Voltage limiting resistor 35 is now in the resonance circuit, to establish the relay holding voltage. Neon lamp 42 is energized through a circuit including a 200,000-ohm resistor 43, conductors 44, 40, relay contacts 39, and conductors 41, 36, and indicates that the blanket is in operation. The blanket will now remain in operation, subject only to the periodic cycling of bimetallic element 26 so long as the temperature of the control layer 13 remains below the cut-off temperature. The resonance of the control circuit is not affected by the operation of the cycling control 21 and relay 25 will hold in. trol layer is accompanied by a drop in impedance in the area of temperature increase, thereby loading the control circuit to cause it to go oif resonance. Resulting from the off-resonance condition, the voltage at relay 25 will drop to a level below that necessary to hold it in and the load circuit will be interrupted. Normal operation may be restored by again momentarily closing the switch 32, provided, however, that the temperature of the thermostatic device has fallen suflicicntly below the control level.

To purposely open the load circuit, there is provided a normally open switch 45, shunted across the relay 25. When switch 45 is closed, the relay coil is deenergized and relay contacts 39 will open.

It will be noted that failure of any of the component parts of the control circuit will de-energize the relay coil 25 by destroying the circuit resonance. The resistor 35 which is placed in series with the load wire 11 when switch 32 is manually closed is effective to reduce the current fiow through the load conductor to-such an extent that even if the switch 32 were held closed continuously in an attempt to circumvent the control circuit, there would be no appreciable heating of the blanket.

The circuit of Fig. 4 operates to shunt out the control relay by conduction between the respective electrodes of A rise in temperature of the con- I the thermosensitive device when the control layer reaches the control temperature. The aforementioned Kearsley control may, of course, be added for normal blanket tem perature control. In contradistinction to the circuit of Fig. 3, the Fig. 4 circuit provides a cycling control in that return from the elevated temperature level to the normal operating level will re-energize the relay and reinstate the load. The thermosensitive device is independent of the load and although its use in a blanket 22a (Figs. 4 and 11) has again been selected for purpose of description, it is obvious that said circuit is applicable to many other types of installation in which it is desired to distribute a thermoresponsive device over a large area such as throughout a coal pile or grain bin. The blanket 22a, which may have the construction described in the aferementioned patent to I. O. Moberg, is provided with a heater wire 50. The thermosensitive element 10 of Fig. 1, or 10a of Fig. 2, is arranged in the heater wire pockets so as to he thermally responsive over the entire heated area. The load 50 is connected to the power source when the energized relay 51 bridges its contacts 52, as will be obvious. The respective conductors 11 and 14 of the thermosensitive device 10 (or the conductors 16, 17 of the device 10a) are in series with each other and with the coil 53 of the relay. A limiting resistor 54 is included in said series circuit. The system may be placed in operation by momentarily closing the switch 55 and may be taken out of operation by closing the switch 56 which shunts out the relay coil 53.

Under usual temperature conditions experienced in normal blanket operation, that is temperatures of the order of 105 R, which corresponds to a medium position setting of the control knob 27 in Fig. 3, the resistance value of the control layer or film is extremely high and the circuit path is through the relay coil 53 in series with the respective conductors of the thermosensitive device. Upon increase of a portion of the thermosensitive device to the cut-off temperature-for example, a portion in a fold of the blanket of Fig. 10, assuming the same to have been left connected to power when in the folded position the drop in resistance or impedance of the control layer along a suitable length will create a shunt path between the control conductors and de-energize the relay 51. The increase in temperature may be gradual but the load circuit will be cut out before the blanket can scorch.

In the Fig. 4 circuit, the following values are typical: load 50, from 65 to 70 ohms; thermosensitive conductors less than 500 ohms each; limiting resistor 54, 12,000 ohms; and relay coil 53, 90,000-ohms impedance.

While a number of alternative embodiments of the invention have been described and illustrated, it is obvious that there are additional variations which fall within the true spirit and scope of the invention. Therefore, it is intended that the invention be limited only as may be necessitated by the scope of the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electric heater wire comprising a pair of spaced metallic conductors, and a composition having a high negative coefficient of thermal impedance between said conductors, said composition consisting essentially of a solid copolymer of 30% to by weight of acrylonitrile, 15 to 70% by weight of an acrylic acid ester, and up to 5% by weight of a surface active agent.

2. An electric heater wire, as claimed in claim 1 wherein the copolymer has an acrylonitrile content of between 35% and 55% by weight.

3. In an electric blanket, a combined heating means and thermosensitive device interposed within a predetermined area of said blanket and having an order of flexibility comparable to that of the blanket comprising a flexible heating conductor, a flexible signal conductor, a substantially uniform, continuous solid layer of flexible copolymer of 30% to 85% by weight of acrylonitrile, 15% to 70% by weight of an acrylic acid ester, and up to of a surface active agent disposed between said heating and signal conductors in intimate surface contact therewith to hold them in spaced relation, said copolymer having a negative temperature impedance coefficient to insulate electrically said conductors, one from the other, at the normal operating temperature of the blanket of around 105 F., and to provide a conductive path therebetween at predetermined elevated temperatures to produce a control effect characterized by current flow between said conductors, and a work circuit adapted for connection to said signal conductor and a power source, said work circuit including work device means responsive to said current flow to disconnect said heating conductor from said power source.

4. An electric blanket, as claimed in claim 3, wherein the copolymer has an acrylonitrile content of between 35% and 55% by weight.

5. A heater comprising a flexible fabric, and a heater wire and control element interposed within said fabric, said heater wire and control element comprising'a flexible electrically insulating core, an electric heat generating conductor helically wound thereon, a substantially uniform, relatively thin layer of a copolymer of 30% to 85% by weight of acrylonitrile, 15% to 70% by weight of an acrylic acid ester, and up to 5% by weight of a surface active agent disposed about said conductor in contact therewith throughout its length, a signal electric conductor helically wound throughout its length upon said layer in surface contact therewith, said heat generating and signal conductors being substantially coextensive with said layer and said copolymer having a negative temperature impedance coefficient to insulate electrically said heat and signal conductors, one from the other, at a normal temperature of around 105 F., and to provide a conductive path between said conductors to pass electrical current of controlling magnitude at elevated control temperatures materially in excess of 105 F.

6. A heater, as claimed in claim 5, wherein the copolymer has an acrylonitrile content of between and by weight.

References Cited in the file of this patent UNITED STATES PATENTS 2,486,241 Arnold Oct. 25, 1949 2,579,375 Eisen Dec. 18, 1951 2,581,212 Spooner et al Jan. 1, 1952 2,688,608 Weinstock Sept. 7, 1954 2,745,943 Safiord May 15, 1956 

1. AN ELECTRIC HEATER WIRE COMPRISING A PAIR OF SPACED METALLIC CONDUCTORS, AND A COMPOSITION HAVING A HIGH NEGATIVE COEFFICIENT OF THERMAL IMPEDANCE BETWEEN SAID CONDUCTORS, SAID COMPOSITION CONSISTING ESSENTIALLY OF A SOLID COPOLYMER OF 30% TO 85% BY WEIGHT OF ACRYLONITRILE, 15% TO 70% BY WEIGHT OF AN ACRYLIC ACID ESTER, AND UP TO 5% BY WEIGHT OF A SURFACE ACTIVE AGENT. 